Thursday, March 14, 2013

Nest defense

Decorative birdhouses are usually not weather-tight and don’t allow for easy cleaning.
 If placed outside, the hole should be blocked to discourage house-hunting birds.

Cat vs. bird: how to give birds an edge
Originally published March 8, 2013 in the Santa Cruz Sentinel

Two summers ago a family of Bewick’s Wrens took up residence in my front yard. They chose to nest in a small birdhouse which I had hung from a plant hanger less than 3 feet above the ground. We have two outdoor cats and I couldn’t believe these birds would mistake a garden ornament for a potential maternity ward. I thought about moving the birdhouse to a safer location, but by then I could hear the urgent peeps of the hungry babies and it didn’t seem wise to move the nursery.

A Bewick’s Wren with a treat for its hatchlings, pauses to make
 sure it’s safe, before darting in and out of the birdhouse
 placed too close to the ground in my front yard.
Mom and dad were tirelessly devoted to their hatchlings and they allowed me to photograph their activity from a distance. They would take turns, perching nearby with an insect in their beak to make sure the coast was clear before darting in and out of the birdhouse in a nanosecond. Anytime our cats got near I chased them away with stern words, hoping they would respect the sanctity of the birds. However, our neighbors’ cats are also frequent visitors to our front yard, so I don’t really know for sure if this wren family survived. One day, they were just gone.  I quickly moved the birdhouse to a less vulnerable location, under the eaves of our house.

I was reminded of the wren family during a recent flurry of online outrage over an “Atlantic” magazine interview, in which a New Zealand economist and environmentalist, Gareth Morgan, suggested that cats should be gradually eradicated from his country for the sake of New Zealand’s native birds and other native species. Morgan’s website, “Cats To Go” explains that New Zealand has the highest per capita cat population on the planet (nearly one cat for every two people) and that these animals have already “contributed to the extinction of nine native bird species.” Therefore, Morgan advises, to save endangered bird populations, pet cats should be neutered and kept indoors, cat owners should not replace their pets when they die, and all stray cats should be caught and euthanized.

Back in the US, Steve Homer of the American Bird Conservancy (a cat owner himself) agreed that cats are a very real threat to native birds. He told ABC News that, “about a third of the birds in the United States are in decline, and cats have been identified as one of the more significant factors in this decline.”  Homer said that habitat loss remains the top threat to birds, but predators rank second.

So all this got me thinking about steps we could all take to create safer bird habit in our own yards. To begin with, we need to better understand the nesting habits of birds.


Different species of birds prefer different types of habitat. Bewick’s Wrens, for example, prefer wooded open forest, farmlands, mixed conifers and deciduous trees. You probably can’t transform your yard from forest to desert, or meadow to marsh, but you can accommodate birds with some simple alternations.
  • Plant native species of plants, shrubs and trees—especially ones that provide food (seeds and berries) or shelter.
  • Leave seed-bearing plants untrimmed a bit longer in the fall and resist the temptation to be too tidy.
  • Sweep dry leaves and branches into piles away from your house for nesting material, and for insect cover, which in turn provide food for the birds.
  • Designer Birdhouses by Richard T. Banks,
    is one of the best birdhouse-building
     books around. Banks gives lots of tips
     for keeping a birdhouse safe from
     predators and provides easy-to-follow
     building instructions for his unique designs.
  • Choose organic or natural products for weed and pest control.


In North America, of the 920 known birds, only about 50 will regularly inhabit a birdhouse. “It may sound like a small number,” writes Richard T. Banks, author of “Designer Birdhouses.” “But the sheer volume of these birds and the steady decline in suitable nesting places is what makes building birdhouses so essential to their survival.” Banks suggests three requirements for building a good birdhouse:

  1. Adequate box and entry-hole size for the intended species
  2. Solid construction for weather-tightness and durability
  3. Security for warding off predators
In his book, Banks provides a chart that gives the optimal dimensions and location of a birdhouse for attracting a particular species of bird. A Bewick’s Wren, for example typically chooses a birdhouse with a 4x4 to 5x5 inch floor, a 1-1/4 to 1-1/2 inch diameter hole that’s 4 to 7 inches above the floor, an interior height of 9 to 12 inches, and placement from the ground of about 5 to 10 feet. Obviously, birds don’t carry measuring tapes, but these are general guidelines.

A functional birdhouse is basically a weather-tight box made from cedar or redwood, held together by galvanized nails or screws, with an entry space or hole, and easy access to the interior for annual cleaning. The outside can be painted or stained to help preserve the wood. A good how-to book such as “Designer Birdhouses” or website such as will provide plans and directions for building a sound, attractive birdhouse.

This simple birdhouse—one
 of dozens placed along trails
 in Christmas Hill Park in Gilroy
—is mounted 8-feet above the
ground on a metal pole which
 is difficult for predators to climb.
 (photo by Suzi Ortiz)
Trees and fence posts are not a good choice for supporting a birdhouse, since cats are good climbers, says Banks. In fact, a safe birdhouse should be placed at least 10 feet away from trees, houses, fences or other jumping-off places. Most birds, except wrens, don’t like the movement of a hung birdhouse, so mounting your birdhouse atop a tall pole or post is the best strategy.

  • Metal Pole:  A ¾-inch threaded, galvanized pipe allows you to screw a matching threaded floor flange to the top of the pole and then attach the birdhouse to the flange. Use a metal fence post spike to hold up the pipe, or dig a hole 18 to 24 inches deep and secure the pipe in quick-set concrete.
  • PVC Tube: A 3-inch-diameter PVC tube can also be fitted with a PVC floor drain flange that fits snuggly inside the tube and serves as the underside mount for your birdhouse. PVC pipe is inexpensive, slick and difficult to climb. Pour about 18 inches of gravel into the tube to make the bottom more stable and attach it to a metal post for stability.
  • Wood post: A 4x4 pressure-treated post will require a baffle to make it secure from cats. A baffle can be a saucer, cone or tube which makes access from the pole impossible. Set the post in a 24-inch deep hole with gravel at the bottom for drainage and add concrete.

And lastly, here’s one my own birdhouse placement tips, taught to me by a family of Bewick’s Wrens: if you have a birdhouse in your yard that’s not in a safe location and you don’t want to move it, cover up the hole to discourage house-hunting birds from settling in.